This invention relates to a radio frequency or optical communication system in which a relay station is used to aid communication among a network of parties, and more particularly to an improvement allowing more efficient use of the available channel resource.
Self-interference cancellation is a theoretically efficient technique for relayed communication between two or more devices involving the transmission of different signals within the same frequency band at the same time. In the example of communication between two devices, such transmission results in a composite signal that includes two signals, one originating from each device. As each device attempts to receive the signal originating from the other device (far signal), it is hindered by interference caused by the signal originating from itself (near signal). Thus, self-interference cancellation works by generating a cancellation signal resembling the device's own near signal and using the cancellation signal to remove at least a portion of the near signal from the composite signal to obtain a signal closer to the desired far signal.
A number of techniques have been developed for self-interference cancellation. However, these techniques have focused on structures which make it expensive and inconvenient to retrofit the large number of existing systems for the capability of self-interference cancellation. In addition, these techniques have failed to take into account various distortions introduced into the transmit signal. As a result, advantages of self-interference cancellation have not been fully realized.
FIG. 1 depicts an existing satellite communication facility with a transmitter system 100 and a receiver system 101 well known in the prior art with no self-interference cancellation capability. The transmitter system 100 comprises a modulator unit 102 and transmitter equipment 104, and the receiver system 101 comprises receiver equipment 108 and demodulator unit 106. Only one of each of these components is shown here for clarity of illustration. It should be understood that there may be more than one of each component in the satellite communication facility. Typically, the modulator unit 102 contains a modulator 112 receiving a transmit (TX) data signal 110 and producing a TX baseband modulated signal 114. An upconverter 116 receives the TX baseband modulated signal 114 and produces a TX interface signal 118 at or near its designated interface frequency. An interface signal as used herein is the signal at the point where a tap can be made in the signal path. The interface frequency, i.e., the frequency associated with the passband of the interface signal, can be any frequency above baseband, namely at a passband such as associated with an Intermediate Frequency (IF) (typically 70 MHz to 2 GHz) or a Radio Frequency (RF) (typically 400 MHz to 30 GHz). An RF frequency is the frequency of signal emission and the IF frequency is typically the frequency of a signal at some location between the baseband processing stage and the signal emission stage. The modulator 102 may produce the TX interface signal 118 using a different method, such as modulating and upconverting all in one step. (Modulating is the process of applying information to a signal.) Typically, the TX interface signal 118, operating at or near the interface frequency, is sent from the modulator unit 102 to a transmitter equipment 104 through a coaxial cable. The transmitter equipment 104 further processes the TX interface signal 118 before transmission to a satellite or other relay station (not shown).
The radio receiver equipment 108 receives a signal from the satellite or other relay station and produces a receive (RX) interface signal 120 at or around an interface frequency. The interface frequency for the receive signal can be the same as or different than the TX interface frequency. Typically, the RX interface signal 120 is sent from the radio receive equipment 108 to the demodulator unit 106 through a coaxial cable. A downconverter 122 aboard the demodulator unit 106 receives the RX interface signal 120 and produces an RX baseband modulated signal 124. A demodulator 126 receives the RX baseband modulated signal 124 and produces an RX data signal 128.
Retrofitting a facility such as an existing satellite communication facility for self-interference cancellation capability has been expensive and inconvenient because current techniques of self-interference cancellation have all focused on structures that require the construction of completely new systems or require tapping into existing systems at inconvenient and/or difficult-to-access locations. For example, a number of these techniques have relied on tapping into the transmit path at the data signal stage for purposes of generating the cancellation signal. Tapping the TX data signal 110 may be impracticable. The TX data signal 110 may be a signal internal to the modulator unit 102. Self-interference cancellation techniques that tap at the output of the modulated signal suffer the same problem. Here, TX baseband modulated signal 114 may also be internal to the modulator unit 102. Efforts to tap such signals may require modifications to circuit boards or other reconfigurations that, if possible at all, are costly and inefficient. Especially given the large amount of relayed communication facility equipment already in service around the world today and the prohibitive expense involved in replacing such equipment or modifying equipment in sealed or otherwise inaccessible enclosures, application of current techniques to retrofit existing equipment for self-interference cancellation capability may be impractical.
In addition, current techniques for self-interference cancellation fail to take into account distortions introduced into the transmit signal. For example, current techniques that do not tap any signals sourced from the transmitter system 100 or tap either the TX data signal 110 or the TX baseband modulated signal 114 simply ignore certain induced distortions such as non-linearity or local oscillation (LO) feedthrough. As a result, the potential advantages of self-interference cancellation techniques and performance of self-interference cancellation systems have not been fully realized. The prior art approaches are illustrated by U.S. Pat. No. 5,280,537 issued Jan. 18, 1994 to Jugiyama et al. and assigned to Nippon Telegraph and Telephone Corporation, U.S. Pat. No. 5,625,640 issued Apr. 29, 1997 to Palmer et al. and assigned to Hughes Electronics, and U.S. Pat. No. 5,860,057 issued Jan. 12, 1999 to Ishida et al. and assigned to Hitachi, Ltd.